Materials with variable permeance to gasses can be desirable for food packaging. Fresh cut fruit and vegetables, and other respiring biological materials, consume oxygen (O2) and produce carbon dioxide (CO2), at rates that depend upon temperature and the stage of their development. Their storage stability depends on the relative and absolute concentrations of O2 and CO2 in the atmosphere surrounding them, and on temperature. For example, CO2 can react with moisture in the package to form carbonic acid. The carbonic acid formed can affect the quality of the produce by accelerating degradation of the produce.
It is also desirable for packaging of food items such as fresh produce to have low moisture permeance under conditions of typical cold storage (low temperatures and low humidity) to retain moisture within the package, thereby retarding desiccation of the produce and maintaining freshness. However, if the packaged produce is exposed to higher temperatures and humidity, such as in a warehouse that is not climate-controlled, increased respiration rates can lead to increased liquid water formation, due to high levels of trapped moisture inside the package, and increased levels of CO2. Packaging that allows moisture vapor to permeate at higher ambient humidity would allow for moisture equilibration with the exterior of the package, thereby helping to minimize formation of carbonic acid by reducing the available liquid water. As a result, the produce would be protected from degradation by carbonic acid, which could allow for extended storage of produce without loss of food quality, even under storage conditions of variable ambient temperatures.
Accordingly, a respiring material is desirably stored in a container whose permeability to O2, CO2 and water vapor is correlated with (i) the atmosphere outside or inside the package, (ii) the rates at which the material consumes O2 and produces CO2 and water and (iii) the temperature, to produce an atmosphere within the container having O2, CO2 and moisture concentrations equal to the optimum values for preservation of the material.
The preferred packaging atmosphere depends on the stored material. For example, some materials, e.g. broccoli, are best stored in an atmosphere containing 1-2% O2 and 5-10% CO2. For other materials, an atmosphere containing 1-2% O2 and 12-30% CO2, e.g. about 15% CO2, is preferred. CO2 concentrations of 10 to 30% can slow the respiration rate of some fruit and reduce the activity of some decay-causing organisms; for example, a CO2 concentration of 20% delays grey mold decay in raspberries and extends their shelf life.
Previous attempts to improve packaging of food items include controlled atmosphere packaging (CAP) and modified atmosphere packaging (MAP). These have been disclosed, for example, in U.S. Pat. Nos. 4,734,324, 4,830,863, 4,842,875, 4,879,078, 4,910,032, 4,923,703, 5,045,331, and 5,160,768, and EP Applications 0351115 and 0351116.
Side-chain crystallizable (SCC) polymers with temperature-variable permeability to O2, CO2 and water vapor as well as packages comprising these SCC polymers or microporous films coated with these SCC polymers have been disclosed. See, e.g., U.S. Pat. Nos. 5,254,354, 6,013,293, 6,210,724, 6,376,032, 6,548,132 and 7,169,451.
The compositions comprise blends of ethylene acid copolymers and organic acids, wherein the combined acid moieties have been at least partially neutralized with metal ions, the metal ions comprising a preponderance of alkali metal ions.